The interactions of complex processes as discussed in Section
3.2 can be analysed with models that incorporate current knowledge at the
process level, including syntheses of experimental results. Process-based models
make it possible to explore the potential consequences of climate variability
for the global carbon cycle, and to project possible future changes in carbon
cycling associated with changes in atmospheric and ocean circulation. Models
can be run with prescribed inputs such as observations of surface climate and
CO2 or the output of climate models. They can also be coupled to
atmospheric general circulation models (Cox et al., 2000; Friedlingstein et
al., 2000), to allow simulation of a wider range of interactions between climate
and the carbon cycle.

Process-based terrestrial models used in carbon cycle studies are (a) terrestrial
biogeochemical models (TBMs), which simulate fluxes of carbon, water and nitrogen
coupled within terrestrial ecosystems, and (b) dynamic global vegetation models
(DGVMs), which further couple these processes interactively with changes in
ecosystem structure and composition (competition among different plant functional
types; Prentice et al., 2000). The treatment of carbon-nutrient interaction
varies widely; for example, some models treat nitrogen supply explicitly as
a constraint on NPP, while others do not. There are currently about 30 TBMs
and <10 DGVMs. Cramer and Field (1999) and Cramer et al. (2001) reported
results from intercomparisons of TBMs and DGVMs respectively. A current international
project, Ecosystem Model/Data Intercomparison (EMDI), aims to test models of
both types against a large set of terrestrial measurements, in order to better
constrain the modelled responses of terrestrial carbon cycling to changes in
CO2 and climate.

Process-based ocean models used in carbon cycle studies include surface exchange
of CO2 with the atmosphere, carbon chemistry, transport by physical
processes in the ocean, and transport by marine biology. The parametrization
of marine biology can be classified as (a) nutrient-based models where the export
of carbon below the surface ocean (approximately the top 50 m) is a function
of surface nutrient concentration, (b) nutrient-restoring models in which biological
carbon fluxes are set to the rates required for maintaining observed nutrient
concentration gradients against dissipation by ocean mixing, and (c) models
that explicitly represent the food chain involving nutrients, phytoplankton,
zooplankton and detritus (NPZD models). In current models, the uptake of anthropogenic
CO2 is controlled mainly by physical transport and surface carbon
chemistry, whereas the natural carbon cycle is controlled by physical, chemical
and biological processes. The Ocean Carbon Cycle Model Intercomparison Project
(OCMIP) compared the performance of four ocean models with respect to natural
and anthropogenic tracers (Sarmiento et al., 2000; Orr et al., 2001), and is
currently undergoing a similar comparison with 13 models and an extended data
set (Orr and Dutay, 1999).